As much as I want to say it was from a performance aspect, better turbo placement for response, I'm guessing the main reason was emissions. Or at least a combination of the two. A shorter exhaust path to the cats allows them to warmup sooner and perform better. They got around it in some of the models with the uppipe cat. The lower end models that had pre-turbo cats should benefit mostly from better turbo response the higher end models that had no pre-cats should get much better emissions.
Turbo should spool faster but the intake piping got much longer. Would make for a great reverse manifold FMIC setup, c'mon aftermarket.